1. Field of the Invention
The present invention relates to a damper, and more particularly to a magneto-rheological fluid damper.
2. Discussion of the Related Art
During the past decade there has been increasing interest in the development of controllable dampers that utilize electro-rheological fluid (ERF) and magneto-rheological fluids (MRF). The possibility of using ERF or MRF based damping devices in various applications has made these controllable devices attractive to designers.
An MRF consists of micron-sized, magnetically polarized particles suspended in a carrier fluid, such as silicon or mineral oils. MRFs are capable of responding to a magnetic field in a few milliseconds. The material properties of an MRF can be changed rapidly by increasing or decreasing the intensity of the applied magnetic field.
Examples of conventional devices are disclosed, for example, in U.S. Pat. No. 3,174,587 to Walton and U.S. Pat. No. 5,277,281 to Carlson et al. Here, the magnetic field is generated by a magnetic circuit which requires a ferrous material for the flow path. Thus, if a ferrous material is not used for the magnetic circuit of an MRF damper, the damper is not operational.
Referring to FIGS. 1 and 2, a conventional MRF damper has an I-shaped magnetic circuit A formed of a ferrous material. Windings of electric wires B produce a magnetic flux C in the magnetic circuit. However, a small gap D is formed in the magnetic circuit A through which the MRF flows according to the displacement of the piston E. In this manner, the magnetic flux C is directed through the MRF to the cylindrical housing F to complete the magnetic circuit path. In the presence of a magnetic field, the ferrous particles in MRP form chains H perpendicular to fluid flow through the passage. Accordingly, increased damping results from the particle chain's resistance to shearing.
However, conventional MRF damper designs have significant limitations. For example, the magnetic path must be formed of ferrous materials. As illustrated in FIGS. 1 and 2, the piston consists of an I-shape magnetic circuit. The magnetic flux lines, that are formed only inside the ferrous magnetic circuit, activate the MRF at the gaps, thereby creating a chain-like formation of MRF particles across the gap to resist the motion of the piston.
Further, the MRF passages must be very small in order to generate a sizable damping force. As shown in FIG. 2, if the dimension of the gap through which the MRF flows is too E large, the chain-like formation of the particles will produce smaller resistance forces. Thus, less resistance to the motion of the piston is obtaining, thereby resulting in smaller damping forces. In contrast, if the gap size is too small, it is difficult to achieve the necessary tolerance in manufacturing the individual components.